The concept of dilution thickening describes a phenomenon whereas the formulation thickens upon dilution with solvent, typically water. Therefore, a formulation is thin enough to get in and out of a container, but becomes sufficiently thick to retain on the hand or skin during application when water is added.
Typically, a dilution thickening composition is one in which a surfactant system has been thickened (e.g., to enhance viscosity) through use of salt (see Canadian Patent No. 2,211,313). Typically in such compositions, a so-called peak viscosity is achieved when salt is first added, and further addition of salt leads to viscosity reduction (“oversalting”). When the composition is diluted, the “oversalted” composition increases in viscosity in the process referred to as “dilution thickening”
Separately from the above phenomenon, the use of so-called “biphasic liquids” is known in the art. These are defined by the fact that the liquid is divided into two phases. Most of these liquids comprise both one layer which is an aqueous layer and a second layer comprising a water immiscible oily liquid (see U.S. Pat. No. 3,718,609 to Weimer; and U.S. Pat. No. 3,810,470 to Olson, Jr. et al., for example).
Biphasic compositions comprising an upper and lower aqueous phase are disclosed in U.S. Pat. No. 6,429,177 to Williams. The compositions comprise (a) 5 to 35% surfactant; (b) 1 to 12% thickener; (c) 4 to 20% polyalkylene glycol and (d) sufficient non-chelating mineral salt (typically “at least 4% or greater) to induce phase separation.
A particular biphasic is one that may be induced by addition of a sufficient quantity of specifically defined polydextrose. Such biphasic is defined, for example, in U.S. Pat. No. 6,787,511 to Patel et al.